The present invention concerns a method for balancing the bandwidth of a dual-mode filter, consisting of a ring resonator produced by strip line technology.
A method for balancing the resonance frequency of a ring resonator is described in the older German Patent Application 198 21 382.4, in which line material is removed with a laser at one or more sites of the strip line ring until a desired resonance frequency is set. An additional strip line is coupled to the strip line ring, in which the resonance frequency of the filter can also be influenced by removing line material for narrowing of the conductor width. It follows from this older application that line material is removed at one or more sites of the strip line ring at which current maxima occur, in order to reduce the resonance frequency. Line material is removed on one or more sites of the strip line ring at which current minima occur, in order to increase the resonance frequency.
Narrow-band notch filters that suppress undesired residual signal of a local oscillator present in the transmitter of a microwave transceiver are required in transmission technology for numerous applications. These filters are supposed to have the highest possible stop-band attenuation in their stop-band range and the lowest possible pass-band attenuation in their pass-band regions, which lie at the frequency below and above the stop-band region. A relatively small transitional region between the pass-band and stop-band region is then desirable. Such notch filters can be produced in the form of ring resonators in strip line technology at very limited demands and cost effectively.
The underlying task of the invention is to provide a method of the type just mentioned, with which not only the resonance frequency of the dual-mode filter consisting of a ring resonator can be balanced, but also the filter bandwidth.
The mentioned task is solved with the features of claim 1 in that the width of the ring line is narrowed at one or more sites of the ring resonator, at which the first of the two modes of the ring resonator has a current maximum, and at one or more sites of the ring resonator, at which the second mode has a current maximum. Owing to the fact that the resonance frequencies of the two modes existing in the ring resonator are balanced separately from each other, it is possible to vary the bandwidth of the filter.
Advantageous modifications of the invention are apparent from the subclaims.
The ring resonator has a line length that corresponds to its average operating wavelength or a multiple of it, so that the two resonance modes existing in the ring resonator have the same resonance frequencies.
An additional strip line is preferably coupled to the ring resonator, via which the waves are coupled in and coupled out of the ring resonator. By changing the strip line width, the upper or lower filter flank can be made steeper, so that a narrower transition is produced between the stop-band region and the pass-band region of the filter on the correspondingly steepened flank. For steepening of the upper filter flank, the strip line can be narrowed at an interval of one-half wavelength, which corresponds to the average operating wavelength of the resonator, from the coupling site between the ring resonator and the strip line. It is then expedient to carry out narrowing on both sides of the coupling site, in each case at a spacing of a half-wavelength. Steepening of the lower filter flank is achieved by widening of the strip line at a spacing of a half-wavelength from the coupling site between the ring resonator and the strip line. The strip line is preferably widened on both sides of the coupling site, in each case at a spacing of a half-wavelength. The widening of the strip line can occur by bonding line pieces laterally onto the strip line.
Removal of the line material for line narrowing can be carried out simply by means of a laser.